Methods for the preparation of the 5-(and or 6)substituted 2-(2-imidazolin-2-yl)nicotinic acids, esters and salts involve the preparation of substituted-2,3-pyridinedicarboxlic acid anhydrides from substituted-2,3-pyridinedicarboxylic acids.
Methods for the preparation of 5-(substituted methyl)-2,3-pyridinedicarboxylic acids are rather limited and those methods that are available may require extensive purification methods to provide high purity 5-(substituted methyl)-2,3-pyridinedicarboxylic acids. For example, in Great Britain Patent No. 2,192,877 5-(substituted methyl)-2,3pyridinedicarboxylic acids are prepared by halogenating 5-methyl-2,3-pyridinedicarboxylates to obtain 5-halomethyl-2,3-pyridinedicarboxylates. The 5-halomethyl-2,3-pyridinedicarboxylates are then reacted with a nucleophile and further reacted with aqueous alkali to obtain 5-(substituted methyl)-2,3-pyridinedicarboxylic acids. This method is not entirely satisfactory because the halogenation reaction produces a mixture that contains 5-halomethyl-2,3-pyridinedicarboxylates, 5-dihalomethyl-2,3-pyridinedicarboxylates and unreacted starting materials. Arduous or time-consuming purification methods are then needed to obtain high purity 5-halomethyl-2,3-pyridinedicarboxylates. If the mixture is not highly purified, the final herbicidal 5-(substituted methyl)-2-(2-imidazolin-2-yl)nicotinic acid, ester and salt compounds are contaminated with herbicidal compounds which have different herbicidal properties than the desired compounds.
Additionally, if the mixture is not highly purified, the nucleophile reacts with the 5-dihalomethyl-2,3-pyridinedicarboxylates to form undesirable 6-( and/or 4) substituted-5-(substituted methyl)-2,3-pyridinedicarboxylates. These undesirable compounds are difficult to separate from the desired 5-(substituted methyl)-2,3-pyridinedicarboxylates and contaminate the final herbicidal 5-(substituted methyl-2-(2-imidazolin-2yl)nicotinic acid, ester and salt compounds.
It is, therefore, an object of this invention to provide high purity 5,6-disubstituted-3-pyridylmethyl ammonium halide compounds which are useful intermediates in the preparation of high purity 5-(substituted methyl)-2,3-pyridinedicarboxylic acids.
It is also an object of the invention to provide a method for the preparation of the 5,6-disubstituted-3-pyridylmethyl ammonium halide compounds and a method for the preparation of 5-(substituted methyl)-2,3-pyridinedicarboxylic acids from said 5,6-disubstituted 3-pyridylmethyl ammonium halide compounds.